1. Field of the Invention
The present invention generally relates to nuclear medicine, and systems for obtaining nuclear medical images of a patient's body organs of interest. In particular, the present invention relates to a novel detector configuration for single photon imaging including single photon emission computed tomography (SPECT) and planar imaging.
2. Description of the Background Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
Single photon imaging, either planar or SPECT, relies on the use of a collimator placed in front of a scintillation crystal or solid state detector, to allow only gamma rays aligned with the holes of the collimator to pass through to the detector, thus inferring the line on which the gamma emission is assumed to have occurred. Single photon imaging techniques require gamma ray detectors that calculate and store both the position of the detected gamma ray and its energy.
Two principal types of collimators have been used in nuclear medical imaging. The predominant type of collimation is the parallel-hole collimator. This type of collimator contains hundreds of parallel holes drilled or etched into a very dense material such as lead. The parallel-hole collimator accepts only photons traveling perpendicular to the scintillator surface, and produces a planar image of the same size as the source object. In general, the resolution of the parallel-hole collimator increases as the holes are made smaller in diameter and longer in length.
The conventional pinhole collimator typically is cone-shaped and has a single small hole drilled in the center of the collimator material. The pinhole collimator generates a magnified image of an object in accordance with its acceptance angle, and is primarily used in studying small organs such as the thyroid or localized objects such as a joint. The pinhole collimator must be placed at a very small distance from the object being imaged in order to achieve acceptable image quality. Pinhole collimators offer the benefit of high magnification of a single object, but lose resolution and sensitivity as the field of view (FOV) gets wider and the object is farther away from the pinhole.
Other known types of collimators include the slant-hole collimator, converging and diverging collimators, and the fan beam collimator. The slant-hole collimator is a variation of the parallel-hole collimator but with all holes slanted at a specific angle. This type of collimator is positioned close to the body and produces an oblique view for better visualization of an organ whose line of sight may be partially blocked by other parts of the body. The converging collimator has holes that are not parallel but instead are focused toward the organ, with the focal point being located in the center of the field of view. The image appears larger at the face of the scintillator using a converging collimator. A diverging collimator results by reversing the direction of the converging collimator. The diverging collimator is typically used to enlarge the FOV, such as would be necessary with a portable camera having a small scintillator. The fan beam collimator is typically used with a rectangular camera head to image smaller organs. The holes are parallel when viewed from one direction and converge when viewed from another direction. The fan beam collimator allows the maximum surface of the crystal to be used to capture imaging data. In most applications, the choice of collimation represents a trade-off between the size of the FOV and the sensitivity and spatial resolution required to properly visualize the target object or organ.
Conventional single photon imaging systems with parallel-hole collimation use large area (on the order of 2000 cm2) monolithic scintillation detectors, and typically have an intrinsic spatial resolution of approximately 3.5 mm FWHM (Full Width Half Maximum). Such detectors are made either of sodium iodide crystals doped with thallium (NaI(TI)), or cesium iodide (CsI). Scintillations within the NaI crystal caused by absorption of a gamma photon within the crystal, result in the emission of a number of light photons from the crystal. The scintillations are detected by an array of photomultiplier tubes (PMTs) in close optical coupling to the crystal surface.
The intrinsic spatial resolution is primarily determined by the size of the PMTs. The design of the parallel-hole collimator (i.e., the length and diameter of the collimator holes) fixes the system resolution, and represents a trade-off between sensitivity (i.e., the number of detected gamma rays) and spatial resolution (i.e., sharpness of the image) of the imaged target object. The system spatial resolution is a quadrature sum of the geometric resolution of the collimator and the intrinsic resolution of the camera. In most clinical imaging studies, the predominant spatial resolution achieved is determined by the geometric resolution of the collimator, and thus there has not been a strong incentive to increase the intrinsic spatial resolution of the gamma camera.
Conventional commercial gamma cameras are designed to minimize cost by using the largest possible size PMTs, and thus achieve an intrinsic spatial resolution of about 3.5 mm FWHM. However, recent detector technology has enabled the design of small gamma cameras with intrinsic spatial resolution of less than 1 mm FWHM. Thus, there exists a need in the art for improvements in collimator technology to take advantage of such increased intrinsic spatial resolution in the development of new commercial gamma cameras.